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The CH340 chip is used by a number of Arduino compatible boards to provide USB connectivity, you may need to install a driver, don’t panic, it’s easier than falling off a log, and much less painful.
(Manufacturer’s Chinese Info Link)
(Manufacturer’s Chinese Info Link)
The following github has up to day pkg files for 1.3, 1.4 and 1.5 at time of writing, thanks to Joshua Wallis for bringing this to my attention…

Direct Download Links (from the github repo above):

See the github link above for installation instructions if you need them.
(V1.3) Download the CH340 Macintosh Signed Driver for Mavericks (10.9), Yosemite (10.10) and El Capitan (10.11) and Sierra (10.12)
Inside the zip file you will find a PDF and a “pkg” file, short version, double click the pkg file, the PDF has some extra information.
OSX Sierra (10.12) Note
A kindly user, Maarten Segers reports
FYI, the driver documented here WILL crash on OSX Sierra.
I can personally not test on MacOS and can not vouch for the above drivers at github, but there you go.
See uninstalling information at the bottom of the page if the driver causes problems for you.
Here is an older version of the Mac driver, NOT FOR 10.12 Sierra

(V1.0) Download the CH340 Macintosh Signed Driver for Mavericks (10.9), Yosemite (10.10) and El Capitan (10.11)
(Manufacturer’s Chinese Info Link)
Drivers are almost certainly built into your Linux kernel already and it will probably just work as soon as you plug it in. If not you can download the Linux CH340 Driver (but I’d recommend just upgrading your Linux install so that you get the “built in” one).
A couple of Mac users have said “the driver crashes my mac Sierra how do I uninstall”.
Firstly regards the crash make sure you installed the 1.3 version of the driver, not the old one. You could download it directly from the Manufacturers Website in case they have issued an update since I wrote this page.
Secondly a quick google search reveals that this is how you uninstall:
cd /Library/Extensions/ls | grep usbserial.kextusbserial.kextsudo rm -R usbserial.kextls | grep usbserial.kextcd /private/var/db/receipts/ls | grep usbserial*codebender.cc.ch34xinstaller.usbserial.pkg.bom codebender.cc.ch34xinstaller.usbserial.pkg.plistsudo rm -r filename`